Date: Mon, 21 May 2018 13:19:01 +0200
From: Dimitry Andric <dim@FreeBSD.org>
To: Thomas Munro <munro@ip9.org>
Cc: freebsd-hackers@freebsd.org
Subject: Re: printf("%m") doesn't generate a warning -- shouldn't it?
Message-ID: <BAD690AA-C51A-4125-8F23-B637758C8EB6@FreeBSD.org>
In-Reply-To: <CADLWmXU__ZRaS=gg_Y18WYbJJgk2Rs065RhkYmAZCFY8mp1Wjg@mail.gmail.com>
References: <CADLWmXU__ZRaS=gg_Y18WYbJJgk2Rs065RhkYmAZCFY8mp1Wjg@mail.gmail.com>
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On 21 May 2018, at 04:38, Thomas Munro <munro@ip9.org> wrote:
>
> As discussed on the PostgreSQL[1] and NetBSD mailing lists[2][3],
> syslog-like printf("%m") is a GNU extension that doesn't generate a
> warning from Clang or GCC on other operating systems even though when
> it doesn't actually work. That's because the __printf__ attribute
> that our __printflike macro in /usr/include/stdio.h expands to
> effectively means "like printf in glibc, allowing %m", not like POSIX
> or our actual libc which just prints out "m" when it sees it.
>
> It'd sure be nice to get a compiler warning on FreeBSD when porting
> software that uses that if it doesn't actually work (or ... to support
> it).
Please submit upstream bug(s) for clang and gcc. It turns out clang has
a -Wformat-non-iso warning flag which should warn about this, but I
can't get it to emit it:
$ cat printf-m.c
#include <stdio.h>
int main(void)
{
printf("error: %m\n");
return 0;
}
$ clang -std=c99 -Wformat-non-iso -c printf-m.c
<nothing>
-Dimitry
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